The present invention relates generally to medicinal drug pumps, and, more specifically, to implantable molecular drug pumps.
U.S. Pat. No. 4,639,244-Rizk et al. discloses one type of electrodiffusion molecular pump for dispensing over time a suitable drug, such as insulin, when implanted in a patient. The pump has no moving parts and includes a selectively permeable diffusion membrane which permits controlled dispensing of drug molecules from an internal reservoir of the pump by diffusion through specifically sized pores in the membrane. Diffusion is a conventional process in which high concentration of molecules in a fluid will migrate to areas of low concentration. The pore size of the membrane is selected for passing the desired molecules, such as insulin, through the membrane at a controlled rate.
However, the drug diffusion rate is substantially fixed, and it is desired to vary the drug rate as required by the needs of the patient in which the pump is implanted. Accordingly, the pump further includes a pair of carbon mesh electrodes disposed on opposite sides of the diffusion membrane which are suitably electrically powered to effect electrophoresis. In this way, one electrode becomes a cathode, and the other electrode becomes an anode, and ionic drug molecules, such as negatively charged insulin, may be additionally driven through the membrane by the electrical field created between the electrodes.
By charging the electrode external of the diffusion membrane positive, and the electrode inside the pump reservoir on the opposite side of the membrane negative, the negative ions, or ionic molecules, may be electrically driven through the membrane with an increased delivery rate over that available solely for diffusion. Alternatively, the external electrode may be charged negative and the internal electrode may be charged positive for retarding diffusion through the membrane to decrease the delivery rate of the drug molecules.
In this way, the drug pump is configured for delivering the drug under diffusion at a substantially constant rate, which may be selectively increased or decreased by powering the electrodes for effecting electrophoresis, with the combination thereof being referred to as electrodiffusion.
Since it is desired to implant the pump in a human or animal patient for long periods, the pump itself should be relatively small and durable, and require little power to operate. However, these different objectives typically require compromises in the design of the pump.
For example, since the electrodes cover both sides of the diffusion membrane, they too must also be porous to permit dispensing of the drug outwardly therethrough. The electrode pores may be provided between the electrically conductive fibers forming the woven electrode mesh. The mesh should be woven tightly with fine fibers for maximizing the surface area of the electrode from which the electrical field is generated. Maximizing the surface area correspondingly decreases the power requirements for generating the electrical field during operation with maximum strength for effecting the electrophoresis migration of the drug ions.
In the above identified patent, the electrodes are formed by crossing small carbon fibers of about 1-10 microns in thickness in a tight mesh. However, testing conducted in recent developments suggests performance inefficiency during electrophoresis due to such mesh configuration.
Furthermore, the diffusion membrane is about 100-200 microns thick with minute micron-size pores therein, and is delicate and fragile. Accordingly, the repeated refilling of the drug reservoir in the pump can substantially shorten the useful life of the membrane when subjected to elevated differential pressure thereacross.
Accordingly, it is desired to provide an electrodiffusion ionic drug pump having an improved configuration for increasing durability and electrophoresis performance thereof.
An electrodiffusion pump includes a housing having an inlet, drug reservoir, and outlet. A diffusion membrane is disposed between the reservoir and outlet for diffusing the drug outwardly therethrough. Internal and external electrodes are disposed on opposite sides of the membrane. The electrodes are porous, and the external electrode is additionally perforate to expose a portion of the membrane at the outlet. The electrodes are electrically powered for driving drug ions through the membrane under electrophoresis to complement diffusion thereof.